等价无穷小的替换
lim [(x+c)/(x-c)]^(x/2)x→∞=lim {[1+ 2c/(x-c)]^[(x-c)/(2c)]}ᶜ·[1+ 2c/(x-c)]^(c/2)x→∞=eᶜ·(1+0)^(c/2)=eᶜ=3c=ln3
